The invention relates to a method of manufacturing a volumetric liquid meter of the oscillating piston type, and also to a volumetric liquid meter manufactured by the manufacturing method of the invention. Such a meter comprises a cylindrical measuring chamber made up of various elements: a bottom portion comprising a side wall and a bottom wall having an inlet port, a top portion that engages thereon comprising a cover having an outlet port, a fixed partition placed between the bottom portion and the top portion, a cylindrical piston placed eccentrically and guided to move inside the chamber by means of the partition occupying a slot of said piston, a filter engaged on said chamber to filter the liquid penetrating via the inlet port, and a sealing gasket for forcing the liquid flow to pass through the measuring chamber while preventing any liquid from flowing directly from an admission fitting of the meter to an exhaust fitting thereof. In general, the method of manufacturing said elements consists in injection molding, using respective molds of shapes that match those of the elements to be made.
In this type of meter, the measuring chamber as shown in FIGS. 1A and 1B is the essential element on which the accuracy of flow rate measurement depends. The measuring vessel is a chamber 30 of cylindrical shape comprising a bottom wall 1, a side wall 2, and a cover 3. The bottom wall 1 and the cover 3 have respective bottom and top cylinders 4 and 5, both of the same diameter, which diameter is smaller than the diameter of the chamber. The two cylinders 4 and 5 are centered on the axis of the chamber. The center of the bottom cylinder has a metal rod 28 on which a roller 6 is engaged. The bottom wall and the cover are provided at least with an inlet port 7 and an outlet port 8 respectively for admitting and for exhausting fluid into and out from the chamber. The chamber 30 also has a stationary partition 9 of rectangular shape separating the inlet port 7 from the outlet port 8. The partition extends radially between the side wall 2 and the bottom and top cylinders 4 and 5, and axially between the bottom wall 1 and the cover 3. The partition 9 is of rectangular shape and has two substantially parallel faces. The partition possesses two main functions, firstly it guides the motion of the piston, and secondly it provides sealing between the inlet and outlet ports. These functions, in particular the function relating to sealing, mean that the partition must be positioned very accurately with very small clearance between the partition and the groove. The bottom wall, the side wall, the cover, and the bottom and top cylinders have a groove 10 in which the partition is received. A piston 11 of cylindrical shape having a diameter smaller than that of the chamber but greater than the diameter of the bottom and top cylinders is positioned eccentrically inside the chamber. At half-height the piston 11 has a plane wall 12 perforated by holes and supporting two studs 13 in its center, one extending towards the bottom wall and the other towards the cover. The wall also has a pear-shaped notch 14 extending radially and positioned eccentrically. The pear-shaped notch opens out via a slot 15 formed over the full height of the piston.
It is also common practice to use a filter 20, as shown in FIG. 2, in association with such a measuring chamber. The filter 20 is cylindrical in shape comprising a bottom wall 19 and a side wall 18. The bottom wall 19 forms a screen for retaining particles of a size larger than the size of the mesh of the screen. The filter 20 is designed to engage on the cylindrical chamber 30, and in particular the screen 19 comes into contact with the bottom wall 1, while the side wall 18 of the filter fits against the side wall 2 of the chamber so that all of the liquid that penetrates through the inlet port 7 initially passes through the filter.
The filter is also provided with a peripheral sealing gasket in the form of a projection. This projection can be an O-ring of semicircular section or a flexible lip. The gasket provides sealing between the measuring vessel and the meter housing (not shown) in which it is disposed, such that the liquid penetrating via the inlet fitting (not shown) of the meter housing can leave via the outlet fitting (not shown) only after penetrating into the measuring vessel.
A flow meter including such a measuring vessel operates on the principle of admitting a given volume of fluid via the inlet port into the piston, which liquid, by communicating its energy to the piston, causes the piston to turn and enables the liquid to be exhausted towards the outlet port. Thus, each revolution of the piston corresponds to a given volume of fluid passing through. The general motion of the piston is oscillating motion, the axis of the piston describing a circle around the axis of the chamber and the slot of the piston sliding along the partition. The motion of the piston in the chamber is guided by the partition engaging in the vertical slot and in the pear-shaped notch, and also by the stud engaging between the roller and the bottom cylinder. The plane wall of the piston, although positioned between the bottom and top cylinders, remains free to move in a plane.
The various elements forming the measuring chamber are made by injection molding, each element being made using a mold of appropriate shape. The injection method used is a conventional method well known to the person skilled in the art. The material used is a thermoplastic.
In order to improve the mechanical characteristics of the various elements, in particular in terms of mechanical strength, coefficient of friction, density, ability to absorb shocks, or indeed leaktightness, it is usual to apply surface treatment to these elements. The particular surface treatment depends on the specific function that the element is to perform. This treatment consists in depositing a surface layer of material, e.g. occupying 100th the thickness of the unmolded element.
It is also known to modify the structure of the element so as to add new functions thereto. Patent No. EP 0 627 614 describes an oscillating piston liquid meter having a partition whose profile is bi-convave. That particular shape serves to optimize the clearance between the piston and the partition so as to reduce the amplitude of shocks. Patent No. WO 93/22631 describes an oscillating-piston water meter in which the surface of the piston wall that comes into contact with the inside surface of the measuring chamber is grooved. The grooving of the piston serves to reduce friction against the wall of the chamber and to reduce the damage generated in the meter by particles entrained in the water.
The methods used for adding new functions or for improving the mechanical characteristics of the various elements constituting the measuring chamber are complicated and lead to methods of manufacturing and assembling the various elements that are complex and expensive.
The object of the invention is to propose a simplified method of manufacture which makes it possible to make the various elements directly using an injection molding technique so that they have improved mechanical characteristics adapted to certain functions of the part, without it being necessary to perform surface treatment after unmolding, and without it being necessary to modify the structure of existing elements.
This object is achieved by a method of the invention consisting in making at least one of the elements of the meter whose mechanical characteristics are to be improved by means of an injection molding technique using a mold of a shape that matches that of the element to be made, the method comprising the following steps:
injecting a first material to form at least a portion of a skin of the element; and then
injecting at least a second material presenting mechanical characteristics that are different from those of the first material, to form the core of the element.
This two-material or multi-material injection method makes it possible to obtain, directly on unmolding, a meter element presenting mechanical characteristics that are improved and specific to additional functions without any need to subject the element to subsequent treatment. In addition, the core and the skin of the meter element have different mechanical characteristics, each of which can perform a different function.
By way of example, the desired functions are as follows:
improving the mechanical strength of the piston by making the piston with a core that is made of a material that is stronger than the material constituting its skin;
improving the coefficient of friction of the piston, of the bottom portion, or of the top portion, by making said elements with a skin made of a material having good friction characteristics and a core made of a material providing mechanical strength or lightness;
lightening the piston, the bottom portion, or the top portion by making said elements with a skin made of a material of low density and a core made of a material that provides mechanical strength, or vice versa;
improving the passage of foreign bodies present in the liquid passing through the meter without damaging the meter by making the piston, the bottom portion, or the top portion with a core comprising a rigid material and a skin comprising a material having good elastic properties;
reducing the noise generated by the meter by making the piston, the bottom portion, or the top portion with a core made of a rigid material and a skin made of a material presenting good shock-absorbing properties; and
integrating the function of sealing the measuring chamber by making a bottom portion or a filter for engaging on the bottom portion that includes an integrated elastomer sealing gasket.